Add zopfli and pigz repositories

Those are required to build 'pigz' as part of the hermetic toolchain.

Bug: 243389125
Bug: 315488400
Change-Id: I35a55a7e172b37f8830a4ca428ec941a31fe4b2c
Signed-off-by: Matthias Maennich <maennich@google.com>

diff --git a/default.xml b/default.xml
index daef581..ffbd588 100644
--- a/default.xml
+++ b/default.xml

@@ -40,4 +40,8 @@
   <project path="external/bazelbuild-rules_pkg" name="platform/external/bazelbuild-rules_pkg" groups="ddk" />
   <project path="external/bazelbuild-rules_python" name="platform/external/bazelbuild-rules_python" groups="ddk"/>
   <project path="external/bazelbuild-rules_rust" name="platform/external/bazelbuild-rules_rust" />
+
+  <!-- dependencies used to build toolchain from sources -->
+  <project path="external/pigz" name="platform/external/pigz" groups="ddk"/>
+  <project path="external/zopfli" name="platform/external/zopfli" groups="ddk"/>
 </manifest>